The present invention generally relates to wireless communication systems, and particularly relates to RF signal modulation.
Wireless communication systems, such as the mobile communication networks that support cellular telephone communications, impose a number of engineering challenges. The desire to support an increasing number of users within a finite radio frequency spectrum, and to provide ever smaller and more capable mobile terminals, challenges both system and component designers. Not all of these challenges are immediately obvious. A layperson might appreciate the difficulty of physically fitting a modern cellular phone into an increasingly small package, but might completely miss the varied challenges associated with increasing its operating efficiency in the quest for greater battery life.
As with many complex undertakings, solving the real problem of interest requires solving a number of related, sometimes ancillary problems. The challenges associated with increasing RF transmitter efficiency are varied and often subtle. For example, the need to increase spectral efficiencyxe2x80x94bandwidth usagexe2x80x94in the interest of supporting more users has prompted development of several digital modulation schemes. These modulation schemes often involve both amplitude and phase or frequency modulation. Transmit amplifiers generally must respond linearly to the amplitude modulation information. This fact suggests the use of a linear transmit amplifier, yet a linearly biased power amplifier in general operates at a lower efficiency than one biased for saturated mode operation.
One approach to increasing transmitter efficiency entails generating separate modulation signals, a phase modulation signal and an associated amplitude modulation signal, for example. By generating the phase modulation signal with a constant-envelope, a saturated mode power amplifier can amplify it without appreciable distortion. The associated amplitude modulation signal can then be used to vary the output signal level from the power amplifier coherently with the phase modulation information contained in the output signal. In this manner, the saturated mode power amplifier may be made to generate a transmit signal with the desired phase and amplitude modulation information. One consequence of xe2x80x9cunwrappingxe2x80x9d the phase or frequency modulation information from the amplitude modulation information can result in relatively high bandwidth phase modulation signals.
The present invention includes methods and apparatus for reducing phase-modulation bandwidth requirements in a radio frequency transmitter. A phase information signal contains a sequence of phase values corresponding to a desired sequence of transmit symbols. Phase transitions between successive phase values that exceed a certain magnitude, or fall within one or more defined ranges, are reduced to lower the bandwidth of the phase information signal. A phase-modulated output signal is generated using the lowered-bandwidth phase information signal. Phase transitions in the phase-modulated output signal corresponding to the reduced phase transitions in the lowered-bandwidth phase information signal are restored to what they would have been without bandwidth reduction. This effectively restores the full value of all phase transitions represented in the original phase information signal to the phase-modulated output signal.
To accomplish the above, an exemplary circuit arrangement includes a signal source, a phase reduction circuit, a phase adjustment circuit and a phase restoration circuit. The phase reduction circuit may operate on successive phase information signal samples to identify phase transitions that exceed a desired magnitude. For example, the phase reduction circuit may be configured to detect phase transitions in the range of 180xc2x0 in the phase information signal. Upon detecting a phase transition within this range, it generates a control signal to the phase adjustment circuit. The signal is timed so that the phase adjustment circuit, which receives the successive phase values of the phase information signal, effectively subtracts 180xc2x0 from the offending phase transition. It may do so by adjusting the involved sample values in the phase information signal. The adjusted phase information signal phase modulates a signal source, which produces a phase-modulated output signal.
The phase restoration circuit couples the phase-modulated output signal to a phase modulation input of a transmit amplifier. It restores the reduced phase transitions in the phase-modulated output signal by adding or subtracting an amount of phase transition associated with the amount reduced in phase reduction operations. It adds or subtracts phase transitions to the phase-modulated output signal at times keyed to operation of the phase reduction circuit. The phase restoration circuit may comprise one or more phase splitters to create one or more phase-offset versions of the phase-modulated output signal. An included switch can then switch between the phase-offset signals to impart the desired amount of phase transition. Preferably, the phase reduction circuit synchronizes phase reduction and restoration functions using one or more control signals.
In general, the phase reduction circuit may be configured to reduce phase transitions that exceed an arbitrary threshold. For example, it may limit the maximum allowed phase transition input to the signal source to 45xc2x0. Thus, a phase transition of, say, 140xc2x0, would be reduced to 5xc2x0 by reducing the transition magnitude by 135xc2x0 (three times 45xc2x0). For this phase transition, the signal source is modulated by a phase transition of only 5xc2x0, rather than the original 135xc2x0 of transition. In conjunction, the phase restoration circuit would be signaled at the appropriate time to switch between the appropriate phase offset signals such that the xe2x80x9clostxe2x80x9d 135xc2x0 of phase transition is restored to the phase-modulated output signal. The phase modulated signal with the restored phase transition is provided to the transmit amplifier for use in generating a transmit signal that includes phase information corresponding to the original phase information signal.